Side band suppression system



Jan. 9, 1940. .1. PLEBA NSKI 2,186,1

I SIDE BAND SUPPRESSION SYSTEM Filed Sept. 8', 1938 DETECTO i4 A. F. R:Q

I. F'. I J4 AMPLIFIER AMPLIFIER J2, IFREQUENO( CHANGER J5 20 I. F'. isAMPLIFIER I MO 35 glmws: E; 44 29 J1 zo 3 24 T AMPLIFI -ob FREQuEMC? f112 ANGER AMPLJFIER f 91 b QZ 30 e an z.

. BY %4/ ATTORNEY.

Patented Jan. 9, 1940 Umreo srn rss SIDE BAND"SUPPRESSION SYSTEM JozefPlebanski, Warsaw, Poland, assignor to Radio Patents Corporation, acorporation of New York Application September s,19ss,seria1 No." 228,923In Poland March 8, 193 8 1 14 Claims. (01.178 44 particularly to animproved circuit for and method of suppressing one of the modulationside bands in the transmission or reception of signals.

of this character.

An object of the invention is to provide a simple and eficient'means anda method for selecting the upperor lower side band of a modulatedcarrier signal. 1 v

A more specific object is to provide a simple and efficient circuitarrangement in a radio re ceiver for eliminating'or minimizinginterference due to overlapping of the modulation side bands of adjacentsignalling channels experienced in the reception of broadcast signals orthe like.

Another object is the reduction or suppression of disturbingheterodyning signals or beat notes in a radio receiver caused by anadjacent carrier wave.

The, above and further objects and-advan-v tages of the invention willbecome more apparent from the following description of several practicalembodiments thereof taken with reference to the accompanying drawingforming partof this specification and wherein I Figure 1 is a circuitdiagram for a radio receiver embodying a side band selecting system inaccordance with the invention,

Figure 2 shows a set of response curves ex planatory of the design andoperation of the circuit according to Figure l, 7' Figure3 illustrates apreferred practical embodiment of a system constructed in accordancewith the invention,

Figures 4 and 5 are theoretical'diagrams explanatory of the design andoperation of Figure 3, Figure 6 illustrates a 1 and 3,

1 Figure 7 shows a diagram explanatory of the efiects and resultsobtained by means of a system according to Figure 6, and

Figure 8 shows a diagram illustrating a furmodification of Figures thermodification of the invention.

Similar reference characters identify similar parts and magnitudesthroughout the different views of the drawing.

Referring more particularly to Figure 1, there is shown an antenna i0connected in a known manner to ground 34 through a series couplingcondenser H and acoupling coil l2. The latter is arranged in inductiverelation with a secondary coil l3 connected tothe input of a frequencychangeror mixer device I4 as provided in' the conventional type ofsuperheterodyne receiver.

In this manner a modulated high frequency signalreceived by the antennais converted into a signal of intermediate or beat frequency portions ofwhich are applied to a pair of separate. intermediate frequencyamplifiers l5 and I6, respec tively. The output signal of'the amplifierI5 is impressed upon the grid of a further amplifying tube l9 includingin its output a resonant circuit comprised of an induction coil 22shunted by a condenser E! in series with a coupling coil 28. Similarlythe output signal of the amplifier I6 is impressed upon the grid of anamplifying-tube including in its output a resonant circuit comprised ofan induction coil E l shunted by a condenser 23 in serieswith a couplingcoil 29. The 1 anodes of tubes l9 and 29 are supplied with highpotential from a source indicated by the plus symbols in a manner wellknown, and the resonant circuits are returned to ground or cathode.

for high frequency through condensers 26 and 27, respectively. The highpotential ends of the resonant circuits are further connected each toone of a pair of fixed contacts of a switch 39 having a movable contactconnected to a detector andv audio frequency amplifier 32 through acoupling condenser 3|. The output of the audio amplifier may serve tosupply a translating device such as. a loud speakerv'i i. The resonantcircuits 2|, 22, 28 and 23, 2Q, 29 are tuned to the signal frequency,that is in the example illustrated the intermediate or beat frequency ofthe receiver.

.By properly adjusting the coupling between the resonant circuits 2|,22, 28 and 23, 24, 29 through the coils .28 and 29, either of themodluation side. bands of the impressed input signal may be selected andapplied-to the detector or audio frequency, amplifier by connecting theswitch 30 in the upper or lower position, respectively. As isunderstood, the received high frequency signal, may be directly appliedto the amplifier tubes I9 and 2d or tuned circuits El, 22 or 23, 24,respectively, i. e. in the manner of a straight high frequency receiverwithout changing to aninterm'ediate frequency as shown in the exampleillustrated.

In order to obtain the above eifect, that is to suppress. either of themodulation side bands in the circuits 2|, 22, 23 and 23, 24, 29 theamplifiers l5 and It or translating channels have to be designed withpredetermined characteristics 'as explained in greater detail in thefollowing.

The amplitude response characteristic of the amplifier l5,that is theamplitude of the output current I input as a function of frequency isshown at A in Figure 2, that is the amplitude response is substantiallyconstant throughout the modulation frequency band. Similarly, the phasecharacteristics of this amplifier is assumed to be constant such asshown at P1 in Figure 2, or in other words no substantial phasedisplacement occurs during the passage of the signals from the input tothe output. The amplifier IS on the other hand is designed to have anamplitude response characteristic similar to the amplifier I5, that isas shown at A in Figure 2, and a phase response characteristic differingsubstantially from the phase response characteristic of the amplifier l5as shown at P2 in Figure 2. According to the latter which represents anideal condition, the time phase of all frequencies below the resonantfrequency f0 (lower side band) is and the phase of all the frequenciesabove the resonant frequency Ju (upper side band) is +90 or, in otherwords, all frequencies of the left side band are shifted by 90 inrespect to the same frequencies in the amplifier l5 and all frequenciesof the right side band are shifted by +90 in respect to thecorresponding frequencies in the amplifier l5. Theoretically thecurrents developed in the two resonant circuits 2|, 22, 28

and 23, 24, 29 may be represented by the following equations:

EgwM

E I Z cos +z) EgR E wM whereby it is assumed that the electromotiveforce developed in the first circuit is equal to E1 sin (wt+ n) and theelectromotive force developed in the second circuit is equal to E2 sin(wt+ 2) and wherein R1 and R2 represent the total loss resistances ofthe respective circuits, M represents the mutual inductance between thecoils 28 and 29,

& 21r

is the signal frequency, in the example illustrated the intermediate orbeat frequency of the receiver and Z is a factor depending on variousdesign constants of the two circuits.

From the above equations it is seen that by using amplifiers with theproper phase characteristics the upper modulation side band will 30 ineither its upper or lower position, either one of the modulation sidebands can be selected and impressed upon the detector and audioamplifier to suit any existing requirements.

The invention by reason of the relative simplicity of the circuit andits adjustment has great advantages over the existing methods of sideband suppression or selection requiring filters with sharp cut-offcharacteristics, the latter being complicated in design and as a resultthereof bulky and costly. In the case of transmitters or modulators, ithas become known to suppress one side band by modulating each of a pairof carrier waves having a quadrature phase relation by a correspondingpair of modulating waves also having a quadrature relation and by addingor subtracting the output products, whereby one of the modulation sidebands is suppressed or neutralized. Systems of this type, however, arealso expensive and complicated both in design and operation dueprimarily to the fact that a 90 phase shift of both the carrier and thecomponents of the modulating signal is required. Especially, in the caseof the latter great diniculties are experienced in equally shifting by90 the phase of all the components in an extended band of modulatingfrequencies such as an audio or video frequency signal band. .Thesedifficulties are completely avoided by the present invention whichmerely requires the dividing of a modulated carrier into two components,passing the components through channels of predetermined phase andamplitude characteristics and exciting a pair of mutually coupledresonant circuits in the manner described hereinbefore.

Referring to Figure 3, there is illustrated a preferred practicalcircuit arrangement for obtaining an amplitude and phase characteristicin the signal channels feeding the coupled resonant circuits. AlthoughFigure 3 is described with reference to a transmitter, it is understoodthat the arrangement shown equally applies to a receiver or any othersystem for translating modulated carrier energy.

Referring more particularly to Figure 3, there is shown at 35 agenerator or oscillator producing high frequency currents. The latterare applied to a modulator 31 of any known type There. is

and coupled to the grid of a'further amplifying.

tube It through a grid coupling condenser 44 and grid leak resistance 44substantially similar to Figure 1.

Contrary to the tube 38, the output circuits of the tube 39 contains aresistance capacity coupling arrangement comprising in the example showna coupling condenser 46 in series with a coupling resistance 46. Asuitable tap point of the latter is connected to the grid of the tube38. The coupled resonant circuits 2|, 22, 28 and 23, 2d, 29 connected inthe output circuits of the tubes l9 and 2t] and the side band selectingswitch 38 are substantially similar to the arrangement shown in Figure1.

Referring to Figure 4 showing diagrams explanatory of the function ofFigure 3, curve A represents the amplitude transmission characteristicof the upper channel determined substantially by the tuned circuit 42,43 while P2 represents the phase characteristic which differs from theideal characteristic in Figure 2 by a more gradual change of the phasefrom -90 to +90= in place of the sudden phase reversal in the idealcondition. Accordingly, the side bands are not completely suppressed asin the case of Figure 2, but substantially weakened w as seen from thecharacteristic curves A and A in Figured. nae tunedcircuit 42, as has alow damping which may be obtained by regen eration through thefeed-backcoil 45 connected in the output circuit of the tube I9 andarranged in inductive coupling relation with the coil 43, the side bandcharacteristics vA and Af' will assume ashape such as shown in Figured,In order to adjust the relative amplitudes of the exciting potentialsimpressed upon the tuned circuits 2|, 22, 28 and Z3, 24, 29,.suitableregulating means are provided such as an input resistance 46 havingfavariable tapconnected to'the control grid of the tube 23 as describedhereinabove.

signed and adjusted so as to produce side band It has further been foundadvantageous to'keep the amplification of the tubes 39 and Ziiat a lowvalue whereby by proper adjustment of the reaction coil 45 andthepotentiometer 35 it is possible to obtain side band characteristicsof varying shape and degree of suppression of the side bands or anydesired part thereof. In arrangements of the type described, de

characteristics as shown in Figures 4 and '5, the latter are symmetricalfor the upper and lower side band in respect Ito-the carrier frequency,whereby either of the side bands may be selected for transmission orreception by means of switch M as shown in Figures 1 and 3. The selectedside band may be applied to any output or utilization circuit connectedto point a in a manner wellunderstood from the above.

A characteristic of the circuit shown: in Fig ure 3 is the fact that ifthe damping of the resonant circuit 42, '43 is decreased one of the sidebands will be reduced to a greater extent while the lower frequencies inthe other side band will be accentuated which may be useful in somecases; Furthermore, by adjusting the system in such a manner that thecarrieramplitude is substantially greater than :the side bandamplitudes-as shown-in Figure 5 (by vary-.

ing the degreeof regeneration through coil 45 and adjustment ofpotentiometer 46') the demodulation in a subsequent detector issubstantially improved especially when employing linear detection.

Referring to Figure 6, there is shown a modlfication of the. inventionespecially suited although not limitatively forsuppression of disturbingheterodyning signals produced by beating between twoadjacent carrierfrequencies. According to this embodiment, means are provided forproducing an additional fixed phase shift between the potentialsexciting the coupled side band suppression circuits through theamplifiers I5 and it. In the example illustrated this is obtainedby theprovision of aiphase shifting circuit:,,arran'gement connected betweenthe frequency changingor mixer stage l4 and the amplifiers l5 and It andcomprising an output transformer 49 of the frequency changerrM having asecondary tuned to the intermediate frequency by a parallel condenser50. The phase shifting arrangement comprisesa pair of series networksconnected acrossthe transformer sec ondary, the first of said-networksbeing comprised of a condenser 5| in series with aresistance 52, thelatter being shunted in the example shown by a variable resistance53,'and the sec- 0nd of said networks comprising an ohmic 'resistance 54in series with'a condenser 55. There are further provided a pair ofamplifying tubes 56 and 5?, theinput'control grid of the formerbeingcon'nected to junction betweenthe resistance 5d and condenser 55and the inputzgrid of the latter being connected to avariable tap pointof the'resistance 52. The amplified output currents of the tubes. 56 and51 are impressed upon the amplifiers i5 and i6 for furtheramplificationand the output terminals of the latter b system of substantially thesame type as shown in Figures 1 and 3,'but omitted in Figure 6 for easeof illustration, "-By means of the network 5i+55,-it is possible toeffect a phase shift between the potentials impressed upon tubes 56 and5'! from 0 to 180 to suit any existing requirements. Thus, when using aphase shift of about 45 between the exciting potentials and employingamplifiers having amplitude and phase characteristics of the, type,shown in Figure 4, the resultant side band characteristics in thecoupled resonant circuits 2!, 22, 28 and 23, 24,

29 will be as shown in Figure 7.' "From the latter it is seen'that theside. band characteristics A and A are no longer symmetrical and that acomplete suppression takes place .fora definite frequency above or belowthe resonant frequency, f0 depending on which of the circuits is usedorthe position of the switch 353. The freand c are connected to a sidebandelimination quency f for whichcomplete'suppressiontakes place can becontrolled by regulating the resistances' 52 and 53, that is by'adjusting both the initial phase shift and relative amplitude of theexciting potentials exciting the coupled resonant It will be evidentfrom the foregoing that the presentinvention providesa simple means andI method for suppressing one side band ina modulated carrier signal andhas great advantages compared with known methods and circuits of sideband. elimination requiring filters with sharp cutoff characteristics'orother circuit arrangejustment. In accordance with the present i11-vention relatively simpletranslating circuits are .ments complicatedboth in construction and adrequired merely having predeterminedamplifrom the above that the-invention may be used with equaladvantagefor jside band suppression on both long and short waves,thereby greatly extending the use and possibilities of this method ofmodulated signal energy transmission.

The curves shown in Figures 4, 5 and 7 are actually plotted inconnection with experiments conducted by applicant with coupled circuits'tude and phase characteristics. It is further seen having a rather highdamping (R= ohms) side band frequencies. The resistances R1 and R of thecircuits were equal to each other and to the mutual coupling reactanceQM. Both coupling circuits were tuned to the carrier frequency of kc.The phase shifting circuit 42, 43 had in order to'avoid a substantialattenuation of the a total loss resistance of 100 ohms in the case Allthe curves shown encompass approximately :10 kc. at both sides of thecarrier frequency Q, The invention has special use for improving theselectivity (if-wireless receivers -by' enabling of Figure l and 10 ohmsin the case of Figure 5. v

itto eliminate interference due-to overlapping side bands of adjacenttransmitting channels energy comprising a pair known as monkey chatteror disturbing beat notes between adjacent channels. Since suchinterferences are usually caused by one of the side bands only, it ispossible to eliminate this interference by selecting either of the sidebands such as by placing the switch 30 in the one or the other position.

The curves shown in Figures 4, 5 and 'I are plotted without taking intoconsideration the overall sensitivity of the circuits. The latter can beobtained by multiplying the amplitudes of the curves shown with thecorresponding amplitudes of the selectivity curves for the remainingparts of the apparatus such as a receiver or transmitter.

The invention may be further used advantageously for providingultra-selective circuits, that is circuits having extremely sharpout-off frequencies at both sides of the resonant frequency. For thispurpose two arrangements of the type disclosed are connected in cascadeand adjusted in such a manner that the cut-off frequency f1 in the firstarrangement is above and the cut-off frequency fiff of thesecond circuitis below the resonant frequency fo aS shown in Figure 8 in such a mannerthat the overall side band characteristic A1 and A2 overlap resulting ina narrow frequency band characteristic with sharp cut-off frequenciesshown by the hatched area in the drawing. In this manner, a resultantband-pass characteristic of extreme selectivity is obtained.

It will be evident from the above that the invention is not limited tothe specific circuits and arrangements shown and disclosed. hereafterfor illustration but that the underlying thought and principle thereofare susceptible of numerous variations and modifications coming withinthe broad scope and spirit of the invention as defined in the appendedclaims.

The specification and drawing are accordingly to be regarded in anillustrative rather than a limiting sense.

I claim:

.1. A translation system for modulated carrier energy comprising a pairof transmitting channels having substantially constant input-outputamplitude response over a range encompassed by the modulation sidebands, means for applying substantially equal portions of said energy tothe inputs of said channels, one of said channels having a substantiallyconstant input-output phase characteristic in dependence upon frequencyand the other channel adapted to change the phase of the modulationfrequencies below and above the carrier frequency between the limits of90 to +90", respectively, resonant circuits tuned to the carrierfrequency connected to the outputs of said channels, mutual reactivecoupling means between said resonant circuits, and a utilization circuitconnected to one of said resonant circuits.

2. A translation system for modulated carrier of transmission channelshaving substantially constant input-output amplitude response over themodulation side band range, means for impressing substantially equalamounts of said energy upon the inputs of said 'channela'one of saidchannels having a substantially constant input-output phasecharacteristic in dependence upon frequency and the other channeladapted to change the phase of modulation frequencies below and abovethe carrier between the limits of -90 to +90, respectively,

reactive coupling means between said resonant circuits, a utilizationcircuit, and switching means for selectively connecting said utilizationcircuit to either of said resonant circuits.

3. A translation system for modulated carrier energy comprising a pairof amplifying channels each comprising at least two amplifying stages incascade, a resonant circuit tuned to the carrier frequency forming acoupling element between successive amplifying stages in said firstchannel, a resistance-capacity network forming a coupling elementbetween successive amplifying stages in said second channel, a pair offurther resonant circuits tuned to the carrier frequency and eachconnected to the output of one of said channels, inductive couplingmeans between said resonant circuits, and a utilization circuitconnected to one of said resonant circuits. 4. A translation system formodulated carrier energy comprising a pair of amplifying channels eachcomprising at least two amplifying stages in cascade, a resonant circuittuned to the carrier frequency forming a coupling element betweensuccessive stages in said first channel, a resistance-capacity networkforming a coupling element between successive stages in said secondchannel, a pair of further resonant circuits tuned to the carrierfrequency and each connected to the output of one of said channels, acoupling transformer interconnecting said resonant circuits, the ohmicimpedances of said last resonant circuits being substantially equal toeach other and to the mutual reactance of said coupling transformer toeffect suppression of the upper and lower modulation side bands,respectively, in each of said resonant circuits respectively, and autilization circuit energized from one of said resonant circuits.

5. A system as claimed in claim 4 including means for adjusting therelative amplitude of the energies impressed upon said last resonantcircuits.

6. A system as claimed in claim 4 including means for reacting upon saidfirst mentioned resonant circuit with currents derived from a point at arelatively higher level of amplification in said first amplifier.

'7. A system as claimed in claim 4 including means for regenerativelyreacting upon said first resonant circuit with currents derived from apoint at a higher amplification level in said first amplifier.

8. A translation system for modulated carrier energy comprising a pairof transmitting channels having substantially constant amplituderesponse over the modulation frequency range to be transmitted, one ofsaid channels being substantially aperiodic and the other channelincluding a resonant circuit tuned to the carrier frequency and adaptedto effect a phase shift between the lower and upper side bandfrequencies between the limits from 90 to +90", respectively, a pair offurther resonant circuits each connected to the output of one of saidchannels,

mutual reactive coupling means between said last resonant circuits, anda utilization circuit energized from one of said last resonant circuits.

9. A system as claimed in claim 8 including means for selectivelyconnecting said utilization 11. A translation system for modulatedcarrier energy comprising an aperiodic amplifier, a periodic amplifiencomprising at least one resonant circuit tuned to the carrier frequency,means for impressing equal portions of the energy to be translated uponsaid amplifiers, a pair of further resonant circuits tuned to thecarrier frequency and connected each to the output of one of saidamplifiers, mutual reactive coupling means between said last resonantcircuits, and a utilization circuit energized from one of said resonantcircults. t 12. In a system as claimed in claim 11 in cluding means foradjusting both the relative amplitude and phase; of the energiesimpressed upon said amplifiers.

13. A translation system for modulated signal energy comprising a pairof transmitting circuits, means for feeding portions! of the energy tobe translated to said circuits, one of said circuits adapted to maintainthe time phase position of the individual frequency components of theenergy transmitted substantially constant and the 'theoutput of one ofsaid transmitting circuits,

mutual reactive coupling means between said resonant circuits, and anoutput circuit connected to at least one of saidrsonant circuits.

14. A system as'claimed in claim 13 including means for initiallyadjusting the relative time phase between corresponding components oflike frequency of saidenergy portions before impression upon saidtransmitting circuits.

' J OZEF PLEBAN SKI.

